1. Field of the Invention
The present invention relates to an epitaxial wafer, a light-emitting element, a method of fabricating the epitaxial wafer and a method of fabricating the light-emitting element. Especially, the present invention relates to an epitaxial wafer and a light-emitting element comprising a convexoconcave portion (an irregularity portion) on a light-extracting surface and a reflective layer comprising plural semiconductor layers, a method of fabricating the epitaxial wafer and a method of fabricating the light-emitting element.
2. Related Art
A conventional light-emitting element comprising an n-type GaAs substrate, an optical reflective layer on the n-type GaAs substrate, an n-type Al0.45Ga0.55As cladding layer on the optical reflective layer, a p-type GaAs active layer on the n-type Al0.45Ga0.55As cladding layer, a p-type Al0.45Ga0.55As cladding layer on the p-type GaAs active layer and a p-type GaAs cap layer on the p-type Al0.45Ga0.55As cladding layer is known, wherein the optical reflective layer comprises a layered structure of n-type AlAs/n-type AlxGa1-xAs which is formed as a chirped structure, in which each thickness of each layer is varied continuously, and relations among a thickness-varying ratio, the number of layers and a crystal-mixing ratio are regulated to obtain a predetermined reflective wavelength width and a predetermined reflectivity (see e.g. JP-A 5-37017).
In the light-emitting element described in JP-A 5-37017, it is possible to improve the output of light because the optical reflective layer reflects the light propagated to the substrate by interference effect of light waves.
In the light-emitting element described in JP-A 5-37017, however, it is difficult to improve the light-emitting output without thickening the optical reflective layer, though it is possible to widen a wavelength width of the reflected light in the optical reflective layer itself. Further, there is a case that a film thickness of the optical reflective layer is required to be increased in order to improve the light-emitting output of the light-emitting element. In such a case, there is a concern that a fabricating cost largely rises up because of the increases of raw materials and the growth time according to the increase of the number of the total layers to be grown up. Furthermore, there is a case that it is impossible to improve largely the light-emitting out even in the light-emitting element comprising both the conventional optical reflective layer and the convexoconcave portion on the surface of the element. Still further, there is a case that a forward voltage is sometimes higher than a value suitable for practical use of the light-emitting element, when the light-emitting element is fabricated from an epitaxial wafer comprising convexoconcave portion on the surface of the element.